Acousto-Optic Tuning of Lasers

ABSTRACT

A semiconductor laser tuned with an acousto-optic modulator. The acousto-optic modulator may generate standing waves or traveling waves. When traveling waves are used, a second acousto-optic modulator may be used in a reverse orientation to cancel out a chirp created in the first acousto-optic modulator. The acousto-optic modulator may be used with standing-wave laser resonators or ring lasers.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/947,067, entitled “Acousto-Optic Tuning of QCLsand ICLs,” filed Mar. 3, 2014, which application is incorporated in itsentirety here by this reference.

TECHNICAL FIELD

This invention relates to tuning quantum cascade lasers and inter-bandcascade lasers.

BACKGROUND

Quantum cascade lasers (QCL) are lasers in which the gain spectrum istypically broader than approximately 5% of the central wavelength of thelaser. In typical configurations (such as Fabry-Perot configurations,exemplified by FIG. 2 without the grating 14), i.e., with one facet ofthe QCL 10 with high reflectivity coating and the exit facet withcontrolled reflectivity anti-reflection coating 12, the QCLs 10 willproduce very high power output, approaching power greater than 4 W at awavelength of approximately 4.6 micrometer. However, as shown in FIG. 1,this output occurs in a bandwidth of approximately 250 nm around thecentral wavelength.

Such broadband operation is acceptable when the precise wavelength orthe bandwidth of the output is not critical, for example, fordirectional infrared countermeasures (DIRCM), targeting, beacon, andillumination applications. On the other hand, there are a significantnumber of very important applications, where the laser output must benarrow band, for example, less than 1 nm wide, and tunable over somewavelength region. These applications include spectroscopy and sensorsfor detection of pollutants, toxic gases and explosives. To obtain a“single frequency” output from a broadband gain spectrum laser such as aQCL, a wavelength dispersive element needs to be introduced within thelaser cavity so that only one selected wavelength can resonate. Suchdispersive elements include diffraction gratings 14 (FIGS. 2 and 3),prisms 18 (FIG. 4) and tunable or otherwise narrow band filters 20 (FIG.5).

A key feature of all of these schemes is that mechanical motion isrequired to tune the wavelength of the laser since the wavelengthselection is dependent on the angle as shown in the FIGS. 1-4. All thetechniques shown in these figures permit tuning of the laser wavelengthover the entire gain spectrum (as long as the round trip optical gainexceeds total cavity losses). But, the tuning is slow because of themechanical motion of a discrete, dispersive element (grating, prism orfilter) and not appropriate for applications calling for ruggedness,such as for sensors that would be deployed in the field, carried bypersonnel or mounted on vehicles. There are many applications thatrequire very rapid tuning because there is a need to obtain a completespectrum of the object under examination in a very short time. Suchapplications include studies of time dependent combustion dynamics andexplosion dynamics, time dependent spectral changes during chemical andbiological reactions, rapid examination of an improvised explosivedevice in standoff detection mode and tracking the release of toxicgases.

There is yet another way of obtaining narrow linewidth output from anotherwise broadband QCL. This is the use of distributed feedbackgrating, which is embedded within the gain structure of the laser. Suchlasers are useful because they are simple to fabricate and are rugged.However, tunability is quite limited around the design wavelength of thedistributed Bragg grating. Typical tuning range for distributed feedbacklasers (DFB) is limited to approximately 5 cm⁻¹ around the designwavelength of the grating. This is but a small fraction of the gainspectrum width of the QCL. The tuning can be carried out either byvarying the QCL drive current or by changing the temperature of the QCL.In either case, no mechanical motion is required. The thermal tuning isslow while the electrical current driven tuning can be relatively fast.However, for obtaining broadband tuning, DFB lasers are inappropriate.

For the foregoing reasons there is a need for rugged, rapid broadbandtuning of quantum cascade lasers.

SUMMARY

The present invention permits rapid broadband tuning of semiconductorlasers, such as quantum cascade lasers and interband cascade lasers,electronically without the use of any mechanical motion for thewavelength selection, by utilizing an acousto-optic modulator, therebyimproving the ruggedness of the laser. The acousto-optic modulator maygenerate traveling waves or standing waves. When using traveling waves,a second acousto-optic modulator may be used in the reverse orientationcompared to the first acousto-optic modulator to cancel out any chirpfrom the first acousto-optic modulator. The acousto-optic modulator canbe used with standing-wave laser resonators or ring lasers.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph of a spectral output from a MWIR high power quantumcascade laser in Fabry-Perot configuration (no internal wavelengthdispersion component).

FIG. 2 shows a schematic of a diffraction grating tuned broadband laser(Littrow configuration).

FIG. 3 shows a schematic of another diffraction grating tuned broadbandlaser (Littman-Metcalf configuration).

FIG. 4 shows a schematic of a prism tuned broadband laser.

FIG. 5 shows a schematic of a narrow bandwidth filter tuned broadbandlaser.

FIG. 6 shows a schematic of an embodiment of an acousto-optic filtertuned quantum cascade laser.

FIG. 7 shows a schematic of another embodiment of an acousto-opticfilter tuned quantum cascade laser.

FIG. 8 shows a schematic of an embodiment of a standing waveacousto-optic modulator tuned quantum cascade laser with optical outputthrough zero order diffraction.

FIG. 9 shows a schematic of another embodiment of a standing waveacousto-optic modulator tuned quantum cascade laser with optical outputthrough zero order diffraction.

FIG. 10 is a graph showing discreet laser wavelength tuning as afunction of acoustic driving frequency for a 1 cm long acousto-opticmodulator.

FIG. 11 is a graph showing discreet laser wavelength tuning as afunction of acoustic driving frequency for a 2 cm long acousto-opticmodulator.

FIG. 12 shows a schematic of an embodiment of a traveling waveacousto-optic modulator tuned quantum cascade laser (Littrowconfiguration).

FIG. 13 shows a graph of a tuning curve for the acousto-opticmodulator-controlled EC QCL setup.

FIGS. 14A-C show measured output spectra for the same QCL outputwavelength at three different acousto-optic modulator frequencies.

FIG. 14D shows measured output spectra from FIGS. 14A-C overlapped ontoa single graph for comparison.

FIG. 15 shows a graph of emission spectrum linewidth dependence onacousto-optic modulator central frequency.

FIGS. 16A-B show switching time data for the acousto-opticmodulator-controlled EC QCL setup.

FIG. 17 shows a schematic defining the beam size D_(B) and sound wavepropagation length to the beam L.

FIG. 18 shows a graph of simulated vs. experimental data for IPAabsorption.

FIG. 19 shows a schematic of an embodiment of a ring laser utilizing anacousto-optic modulator.

FIG. 20 shows a schematic of another embodiment of a ring laserutilizing an acousto-optic modulator.

FIG. 21 shows a schematic of another embodiment of a ring laserutilizing an acousto-optic modulator.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of presently-preferred embodimentsof the invention and is not intended to represent the only forms inwhich the present invention may be constructed or utilized. Thedescription sets forth the functions and the sequence of steps forconstructing and operating the invention in connection with theillustrated embodiments. It is to be understood, however, that the sameor equivalent functions and sequences may be accomplished by differentembodiments that are also intended to be encompassed within the spiritand scope of the invention.

Emission wavelength of a single Fabry-Perot (FP) QCL chip with ananti-reflection-coated back facet is controlled with a dispersiveelement located outside the laser cavity, i.e. using the external cavity(EC) approach. However, in contrast to traditional external cavity QCLswith a moving grating for example, the rapid wavelength tuning in thepresent invention is achieved using an electrically controlledacousto-optic modulator (AOM) 106. Compared with the DFB configuration,the present invention provides continuous tunability of the lasingwavelength over the entire gain bandwidth of the QCL; and compared withEC QCLs with a grating, which do provide continuous and broadtunability, the present invention has no moving parts.

An AOM 106 comprises a transparent material 107 having a piezoelectrictransducer 112 attached at one end and acoustic absorber 114 attached atthe opposite end. The piezoelectric transducer 112 creates a sound wavethat is propagated through the transparent material 107 towards theacoustic absorber 114. In particular, high-frequency acoustic wave inAOMs 106 may be generated in a transparent material 107 (germanium incase of long wavelength infrared (LWIR) region) and this acoustic waveforms an index grating. In some embodiments, two AOMs 106, 108, withopposed travelling acoustic waves, may be used so that the frequencychirp introduced by the first AOM 106 is cancelled by the complementarychirp introduced by the second AOM 108. In other words, the second AOM108 comprises a transparent material 109 having a first end and a secondend, a second piezoelectric transducer 118 at the first end and a secondacoustic absorber 116 at the second end, wherein the orientation of thesecond AOM 108 is reversed compared to the first AOM 106 as shown inFIGS. 6 and 7.

The geometry using two AOMs 106, 108 permits continuous tuning of theQCL (as opposed to discrete tuning in steps). The incident light wavesemitted by the laser are deflected by the travelling acoustic gratingscreated in the AOMs. The angle of the deflection is controlled by thechoices of optical and acoustic wavelengths according to equation 1below:

$\begin{matrix}{{\sin \; \theta_{B}} = \frac{\lambda_{0}v_{a}}{2{nV}_{a}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where θ_(B) is the Bragg diffraction angle, λ₀ is the free space opticalwavelength, n and V_(a) are the refractive index of and the acousticvelocity in the AO material, respectively, and v_(a) is acousticfrequency. The direction of the lasing is determined by the laserresonator cavity so the output laser wavelength can be electronicallyselected by changing the acoustic frequency of the AOM 106, thuschanging the Bragg angle condition for the selected wavelength.Deflection efficiency of approximately 90 percent, comparable to that oftraditional diffraction gratings, has been demonstrated for LWIR AOMs.The response time of the modulators is determined by the transit time ofthe acoustic wave across the material 107 and is consistent with highrepetition rate measurements. Notice that once the system is aligned,there is no mechanical motion involved for tuning the wavelength of theoutput. The AOMs 106, 108 are aligned within the cavity and tuning ofthe output wavelengths is achieved by changing the driving acousticfrequency of the modulators according to Equation 1.

The deflection efficiency depends on modulator radiofrequency (RF) powergenerated by the radiofrequency generators 125, 126. The higher the RFpower, the higher the efficiency, leading to higher feedback strength.

Two possible systems geometries, the one shown in FIG. 6 and another onesimilar to the Littman-Metcalf geometry for traditional EC QCLs (FIG. 7)are possible, among others. An embodiment of the broadly tunable quantumcascade laser is shown in FIG. 6. The laser comprises a gain medium 102,at least one collimating lens 104, at least one acousto-optic (AO)modulator 106, and at least one highly reflective mirror 110.

In the Littman-Metcalf configuration, the output beam 124 exits from themodule through the zero order diffraction of the grating created by theAOM 106. The configuration in FIG. 7 has some advantage over that inFIG. 6. Using the undiffracted beam 120 (or the zero order diffractionbeam in the traditional grating nomenclature) has the advantage of usingone less collimating lens 104 and associated advantage of reduced effortfor aligning two lenses. The removal of one lens also reduces theoverall cavity losses, thus will lead to a broader tuning range for thesame QCL chip. Therefore, as shown in FIG. 7, a collimating lens at oneend can be replaced with a high reflective coating 130 at the gain media102 of the QCL at that end.

Another variation of the configurations shown in FIGS. 6 and 7 ispossible if continuous tuning is not required as may be the case whenthe tunable laser output interacts with a broad spectrum absorber, suchas with chemical warfare agents, explosives vapors, and large moleculetoxic industrial chemicals. This alternate variation, shown in FIGS. 8and 9 correspond to the two configurations in FIGS. 6 and 7,respectively. The primary difference arises from the use of a standingacoustic wave (as opposed to the traveling acoustic waves in FIGS. 6 and7). For example, the acoustic modulator 106 may not have an acousticabsorber 114 at one end. The diffraction of the laser beam from thestanding acoustic wave is not accompanied by a frequency chirp and thusa single resonant AOM 106 can be used, leading to substantialsimplification of the optical setup and reduction in cost, as well.

As mentioned above, this configuration does not permit continuouswavelength tuning of the QCL because the standing wave nature of theacoustic grating requires an acoustic resonance in the modulator andthus only discrete acoustic wave frequencies are permitted, determinedby the length of the AOM element. The acoustic resonance condition iswritten as shown in Equation 2 below:

Nλ _(ac)=2L  Equation 2:

where N is the number of acoustic resonances in the AOM 106, λ_(ac) isthe acoustic wavelength and L is the length of the AOM 106. Thus thediscrete permissible acoustic frequencies, v_(ac), for diffraction ofthe light beam are given by Equation 3 below:

$\begin{matrix}{v_{ac} = \frac{N\; v_{ac}}{2L}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

where v_(ac) is the acoustic velocity in the AO material 107. FIGS. 10and 11 show calculated discrete frequencies that are possible in a GeAOM of 1 cm and 2 cm lengths, respectively.

Thus, for situations where continuous tuning is not required, thestanding wave AOM can provide an alternate and simpler solution sinceeven for a 1 cm long AOM, over 100 discrete wavelengths can be obtained.

Overall, the invention has several important advantages, over thegrating, prism or filter based EC QCL, for practical field applications.For example, the tuning speed is improved. Typical modulation bandwidthfor commercially available AOMs is in the range of tens MHz. Therefore,the required switching time between arbitrary wavelengths of under 1microsecond can easily be achieved. This switching time is at leastthree orders of magnitude faster than that for EC QCLs with a grating.

The flexibility of the wavelength control with AOMs also offers aninteresting option of multi wavelengths operation when AOM wave iscomposed of several discrete frequencies. Each of the sound wavefrequencies in this case will force the laser to operate at thecorresponding optical frequencies. The multi wavelength operation can beimportant in spectroscopy when several spectral lines need to be trackedat the same time.

Also, the ruggedness is improved. In contrast to EC QCLs with a grating,wavelength tuning mechanism does not require any mechanical motion. Themodule can therefore be ruggedized to meet the most stringentrequirements for field applications.

In addition, the yield (cost) is improved. The broadly tunable quantumcascade laser comprises a single QCL chip. The standard single-QCLfabrication process has a relatively high yield, exceeding 50 percentfor watt-level LWIR devices. In case of QCL arrays, yield, however,quickly drops with increase in number of elements. For comparison, evena 10-element array processed from the same material would have a yieldof less than 1 percent. Number of elements in a DFB QCL array requiredto cover the 7 to 11 micrometer tuning region can be as high as onehundred. As a consequence, DFB QCL arrays are projected to have a verylow yield, making this technical approach impractical.

With regards to epi-down mounting, the present invention is compatiblewith QCL epi-down mounting that is now commonly employed to lower activeregion temperature of high average power devices. QCL arrays, on theother hand, are typically mounted epi-up to preserve individualelectrical control for each of the emitters in the array. Overheating ofthe epi-up mounted array will make the goal of reaching high averageoptical power significantly more difficult, or even impossible.

With regards to reliability, all of the components in the proposedmodule have been in commercial use for some time and have proven theirlong-term reliability. The high power FP QCLs have been in commercialproduction for nearly a decade and the AOMs have been commerciallydeployed for several decades. Thus, the two critical components in theproposed configuration, namely FP QCLs and AOMs, represent maturetechnologies.

AOMs have been successfully used for rapid and random access discreettuning of CO₂ lasers on their 9.6 micrometer and 10.6 micrometer laserbands and continuous tuning of Ti: sapphire lasers. Research alsoprovides earlier data on tuning of dye lasers using acousto-opticmodulators. However, because of the only recent development of QCLs,acousto-optic tuning of QCLs has not been attempted.

This technology may be applicable to the interband cascade lasers aswell.

EXAMPLES

Reported is the first operation of tunable external cavity quantumcascade lasers with emission wavelength controlled by an AOM. Along-wave infrared quantum cascade laser wavelength is tuned fromapproximately 8.5 micrometer to approximately 9.8 micrometer when theAOM frequency is changed from 41 MHz to 49 MHz. The laser delivered over350 mW of average power in the center of the tuning curve in a linewidthof 4.7 cm⁻¹. Measured wavelength switching time from the edge of thetuning curve to the center of the tuning curve is less than 1microsecond. Initial spectral measurements of infrared absorptionfeatures of gaseous isopropyl alcohol were carried out, whichdemonstrate a capability of obtaining complete spectral data fromapproximately 8.5 micrometer to approximately 9.8 micrometer in lessthan 20 microseconds. The demonstration paves a way for a new generationof tunable QCLs providing ruggedness, fast tuning and high powercapability in the infrared spectral region.

The AOM 106 tuned QCL configuration is shown in FIG. 12. Thepiezoelectric transducer 112 of the AOM creates a traveling acousticwave in the germanium crystal. The sound wave, in turn, produces aspatial periodic variation in refractive index of the crystal, i.e. itcreates a phase grating. The undiffracted optical beam 120 incident onthe AOM 106 experiences a wavelength dependent deflection. Thediffracted beam 122 subsequently reflects back from the mirror 110 tothe AOM 106 and couples back into the gain medium 102, completing thewavelength-dependent feedback loop. The output optical beam 124 from thegain medium 102 emerges from the other side of the laser.

The period of the acoustic grating is electronically controlled, whichallows for the rapid change of the diffracted wavelength. Typicalmodulation bandwidth for commercially available AOMs is in the range oftens of MHz. Hence, the required switching time, between arbitrarywavelengths, of under 1 microsecond can be achieved. This switching timeis at least five orders of magnitude faster than that for EC QCLs with agrating. In addition, in contrast to EC QCLs with a grating, wavelengthtuning mechanism does not require any mechanical motion. The module cantherefore be ruggedized to meet the most stringent requirements forfield applications.

The frequency of the AOM used in this example was adjustable in therange from 35 MHz to 55 MHz. FIG. 13 shows a measured tuning curve for a9 micrometer QCL, with design described in A. Lyakh, R. Maulini, A.Tsekoun, R. Go, and C. K. N. Patel, “Multiwatt long wavelength quantumcascade lasers based on high strain composition with 70% injectionefficiency”, Opt. Expr. 22, 24272 (2012) (which is incorporated here bythis reference), operating in a quasi-CW mode (350 nanosecond pulses and50 percent duty cycle) as AOM frequency was changed from 41.7 MHz to48.5 MHz (approximately ⅓ of the available frequency range). AOM inputpower was fixed at 35 W during the testing. The emission wavelength forthe EC laser tuned from 1020 cm⁻¹ (9.8 micrometer) to 1170 cm⁻¹ (8.5micrometer). The tuning range available from a single laser can beincreased using broader gain QCL chips.

Measured QCL emission spectrum linewidth for AOM central frequency of 45MHz was 4.7 cm⁻¹. The linewidth can be changed by aligning the system sothat it operates at another AOM central frequency (different Braggcondition). Employment of a higher AOM frequency will result into alarger number of illuminated acoustic grating periods and, therefore, toa proportionately narrower linewidth. This effect is illustrated inFIGS. 14 and 15. The increase in the central AOM frequency from 35 MHzto 55 MHz leads to a reduction in emission linewidth from 5.7 cm⁻¹ to3.9 cm⁻¹.

The response time of the change in the optical wavelength with a changein the AOM frequency has two components: (1) propagation time of theacoustic wave from the acoustic transducer to the edge of the opticalbeam going through the AOM, and (2) the propagation time for theacoustic wave across the optical beam. The first “delay” is the latencytime and does not represent the response time of the change the opticalwavelength when AOM frequency is changed. The latency time can beshortened, almost arbitrarily, by reducing the distance between theacoustic transducer and the position of the optical beam. The actualresponse time, therefore, is determined by the acoustic wave transittime across the optical beam. For the present case, the time it takes anacoustic wave to propagate from the piezoelectric transducer across thegermanium material to the edge of the optical beam is t₁ and time forthe acoustic wave to cross the optical beam is t₂. The time t₂ is theactual response time of the AOM for changing the optical wavelength. Asmentioned above, t₁ can be shortened to almost zero. However, there arelimitations on how short t₂ can be. If t₂ is made too short by makingthe optical beam diameter small, the optical wave will interact with afewer number of acoustic waves and therefore the linewidth of the outputwill increase. The linewidth of the optical output can be reduced bymaking the optical beam diameter larger, but that occurs at the expenseof the response time. The optimal linewidth/response time balance isapplication driven.

FIGS. 16A and 16B show measured response time of the system as AOMfrequency switches from 35 MHz (outside of the tuning curve) to 45 MHz(center of the tuning curve). FIG. 14B shows the AOM frequency controlsignal and FIG. 14A shows the optical signal from the laser, whichconsists of pulses of duration 350 nanosecond pulses with a pulserepetition period of approximately 700 nanoseconds.

AOM frequency is initially equal to 35 MHz, outside of the tuning curveof the laser. The optical signal from the laser is at zero level sincethe Bragg condition is not satisfied anywhere in the laser gainspectrum. AOM frequency is abruptly changed to 45 MHz at time t=0. AOMfrequency of 45 MHz corresponds to the peak of the gain curve. It takesapproximately t₁=L/v_(s)=1.25 microseconds for the acoustic signal withthe new frequency to reach the area where the optical beam is incidenton the crystal (v_(s) is sound velocity and L is defined in FIG. 17).When the sound signal with the new frequency reaches that area, theoptical signal starts growing. It reaches its maximum value at t₂=550nanoseconds+t₁, in a good agreement with the calculated travelling timeacross the beam area t₂=D_(B)/v_(s)=3.4 millimeter/(5.5millimeter/microsecond) approximately 600 nanoseconds (FIG. 16A). TheAOM frequency switches back to 35 MHz at t=3 microseconds. The processis repeated: it takes about t₁+t₂=1.8 microseconds for the laser tocompletely shut down (FIG. 16B).

As mentioned above, the latency time t₁ is proportional to L and can bealmost arbitrarily minimized by designing the AOM so that thepiezoelectric transducer generating the sound wave is positioned closerto the beam area (L in FIG. 17 as short as possible). Time t₂, on theother hand, is fixed for the same choice of material and beam size. Itis, however, already well below 1 microsecond. Thus, if the AOMfrequency was changed between two values, for example, f₁ and f₂, thelaser wavelength will switch between two corresponding values in a timeless than 1 microsecond, the latency time notwithstanding.

To demonstrate the rapid spectral measurement capability for theAOM-controlled EC QCLs, a transmission absorption spectrum of isopropylalcohol vapor that has a broad infrared absorption spectrum. Themeasurement was done in the sweep mode when acousto-optic frequency wasvaried from 42 MHz to 48 MHz. FIG. 18 shows the comparison between anIPA spectrum generated using the HITRAN database and collectedexperimental data. The experimental data were captured in a single AOMfrequency sweep from 42 MHz to 48 MHz that took less than 20microseconds.

In conclusion, the first data for AOM-controlled external cavity QCLs ispresented. These devices offer the advantage of very fast tuningcapability with spectral measurement time of under 20 microseconds. Theconfiguration does not involve any moving parts and therefore can beruggedized for demanding field applications.

The present invention may also be applied to ring laser geometry asshown in FIGS. 19-21. The ring laser configuration has been extensivelystudied in the near infrared lasers. Compared to traditional standingwave cavity lasers, described in other parts of this patent application,ring cavity lasers can operate in either of the counter propagatingdirections or both. This capability offers advantages including thecapability of unidirectional laser oscillation with at least thefollowing advantages: (1) ring laser operating in a unidirectional mode,eliminates the spatial hole burning effect present in traditional cavitylasers and extracts power from the gain medium uniformly, making ithomogeneous, which increases mode competition between adjacentlongitudinal modes and affords the possibility of single mode operationeven at high power outputs; (2) uniform extraction of power withoutspatial hole burning results in higher single mode power output comparedto that possible with standing wave configuration; (3) ring cavityoperation is less sensitive to small misalignments; and (4) ring cavitylaser operated in a unidirectional mode can be less sensitive to opticalfeedback from external elements.

As shown in FIGS. 19 and 20, the ring laser geometry may comprise thegain medium 102 of the laser having anti-reflection coatings 132 a, b atopposite ends of the gain medium 102, a pair of collimating lenses 104,an AOM 106 powered by a radiofrequency generator 125, and a plurality ofhighly reflective mirrors 110 a-c. The highly reflective mirrors arepositioned so that one mirror 110 a receives the diffracted beam 122from the AOM 106 and deflects the diffracted beam 122 to the secondmirror 110 b positioned on the opposite side of the gain medium 102,which in turn deflects the diffracted beam 122 to the third mirror 110c, which deflects the diffracted beam 122 through one of the collimatingmirrors 104 and back to the gain medium 102. In some embodiments, theAOM 106 may have an acoustic absorber 114 opposite the piezoelectrictransducer 112 so as to generate a traveling acoustic wave through thematerial 107 (FIG. 19). In other embodiments, the AOM 106 may not havethe acoustic absorber so as to create a standing wave (FIG. 20).

In some embodiments, as shown in FIG. 21, the laser may only have a highreflective coating 130 at one end of the gain medium 102 and only onecollimating mirror 104 at the opposite end of the gain medium 102. Thebeam is emitted from one end of the gain medium 102 and passes throughthe AOM 106. The beam is diffracted through the AOM 106 and is directedto the first mirror 110 a. Through a series of deflections off of theother mirrors 110 b, 110 c, the diffracted beam 122 returns to the laserat the opposite end having the high reflective coating.

In embodiments utilizing travelling waves, a second AOM 108 may be usedin the opposite orientation relative to the first AOM 106 so that thetravelling waves from each AOM 106, 108 travel in opposite directions sothat the frequency chirp created in the first AOM 106 is cancelled outby the complimentary chirp in the second AOM 108.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention not be limited by this detailed description, but by the claimsand the equivalents to the claims appended hereto.

1. An infrared quantum cascade laser, comprising: a. a quantum cascadelaser gain medium having a first end and a second end; b. a firstcollimating lens adjacent to the first end of the gain medium andreceiving an undiffracted output beam from the gain medium defining afirst path; c. a first end of the gain medium having an anti-reflectioncoating facing the first collimating lens; d. a first acousto-opticmodulator adjacent to the collimating lens and opposite the gain mediumand fixed in place, the first acousto-optic modulator receiving theundiffracted output beam and subsequently emitting a diffracted beam; e.a second acousto-optic modulator through which the diffracted beampasses; and f. a first highly reflective mirror positioned to receivethe diffracted beam from the second acousto-optic modulator, and reflectthe diffracted beam back to the second acousto-optic modulator to thefirst acousto-optic modulator and back to the gain medium to amplify awavelength tuned output beam, wherein the acousto-optic modulatorradiofrequencies are electronically controlled to produce the diffractedbeam at a desired optical wavelength, g. wherein the first acousto-opticmodulator comprises: i. a first transparent medium having a first endand a second end opposite the first end, the first transparent mediumhaving anti-reflection coatings at the optical wavelengths of interest;and ii. a first piezoelectric transducer at the first end of the firsttransparent medium, the first piezoelectric transducer being controlledby a first radiofrequency generator, wherein the first acousto-opticmodulator has discrete acoustic frequencies, v, characterized by theequation v=(NV)/2 L, where N is a number of acoustic resonances, V is anacoustic velocity, and L is a length of the first acousto-opticmodulator, and h. wherein the second acousto-optic modulator comprises:i. a second transparent medium having a first end and a second endopposite the first end, the second transparent medium havinganti-reflection coatings at the optical wavelengths of interest; and ii.a second piezoelectric transducer at the first end of the secondtransparent medium, the second piezoelectric transducer being controlledby a second radiofrequency generator, iii. wherein the firstacousto-optic modulator is oriented oppositely compared to the secondacousto-optic modulator so that the second acousto-optic modulatorcancels out a Doppler shift of an optical radiation caused by the firstacousto-optic modulator.
 2. A quantum cascade laser, comprising: a. aquantum cascade laser gain medium having a first end and a second end;b. a first collimating lens adjacent to the first end of the gain mediumand receiving an undiffracted output beam from the gain medium defininga first path; c. a first end of the gain medium having ananti-reflection coating facing the first collimating lens; d. a firstacousto-optic modulator adjacent to the collimating lens and oppositethe gain medium and fixed in place, the first acousto-optic modulatorreceiving the undiffracted output beam directly from the collimatinglens and subsequently emitting a diffracted beam along a second path;and e. a first highly reflective mirror positioned to receive thediffracted beam and reflect the diffracted beam back along the secondpath to the first acousto-optic modulator through the first path andback to the gain medium to amplify a wavelength tuned output beam,wherein the acousto-optic modulator is electronically controlled toproduce the diffracted beam at a desired wavelength.
 3. The quantumcascade laser of claim 2, wherein the first acousto-optic modulatorcomprises a transparent medium having a first end and a second endopposite the first end, and a piezoelectric transducer at the first end,the piezoelectric transducer being controlled by a radiofrequencygenerator.
 4. The quantum cascade laser of claim 3, further comprising ahighly reflective coating adjacent to the second end of the gain medium.5. The quantum cascade laser of claim 3, further comprising a secondcollimating lens adjacent to the second end of the gain medium throughwhich the wavelength tuned output beam is emitted.
 6. The quantumcascade laser of claim 3, wherein the first acousto-optic modulatorcomprises an acoustic absorber at the second end of the firstacousto-optic modulator.
 7. The quantum cascade laser of claim 6,further comprising a highly reflective coating adjacent to the secondend of the gain medium.
 8. The quantum cascade laser of claim 7, furthercomprising a second acousto-optic modulator in between the firstacousto-optic modulator and the first highly reflective mirror, thesecond acousto-optic modulator comprising a transparent material havinga first end and a second end, a second piezoelectric transducer at thefirst end and a second acoustic absorber at the second end, wherein anorientation of the second acousto-optic modulator is reversed comparedto the first acousto-optic modulator.
 9. The quantum cascade laser ofclaim 6, further comprising a second collimating lens adjacent to thesecond end of the gain medium through which the wavelength tuned outputbeam is emitted.
 10. The quantum cascade laser of claim 9, furthercomprising a second acousto-optic modulator in between the firstacousto-optic modulator and the first highly reflective mirror, thesecond acousto-optic modulator comprising a transparent material havinga first end and a second end, a second piezoelectric transducer at thefirst end and a second acoustic absorber at the second end, wherein anorientation of the second acousto-optic modulator is reversed comparedto the first acousto-optic modulator.
 11. The quantum cascade laser ofclaim 3, further comprising a plurality of highly reflective mirrors toform a ring laser resonator.
 12. The quantum cascade laser of claim 11,further comprising a highly reflective coating adjacent to the secondend of the gain medium.
 13. The quantum cascade laser of claim 12,wherein the first acousto-optic modulator comprises an acoustic absorberat the second end of the first acousto-optic modulator.
 14. The quantumcascade laser of claim 13, further comprising a second acousto-opticmodulator in between the first acousto-optic modulator and the firsthighly reflective mirror, the second acousto-optic modulator comprisinga transparent material having a first end and a second end, a secondpiezoelectric transducer at the first end and a second acoustic absorberat the second end, wherein an orientation of the second acousto-opticmodulator is reversed compared to the first acousto-optic modulator. 15.The quantum cascade laser of claim 11, further comprising a firstanti-reflective coating adjacent to the first end of the gain medium,and a second anti-reflective coating adjacent to the second end of thegain medium.
 16. The quantum cascade laser of claim 15, furthercomprising a second collimating lens at the second end of the gainmedium.
 17. The quantum cascade laser of claim 16, wherein the firstacousto-optic modulator comprises an acoustic absorber at the second endof the first acousto-optic modulator.
 18. The quantum cascade laser ofclaim 2, wherein the quantum cascade laser is an infrared quantumcascade laser.
 19. A method for rapid and continuous broadband tuning ofa quantum cascade laser electronically without the use of mechanicalmotion to select a lasing wavelength, comprising: a. generating a beamfrom a laser cavity of the quantum cascade laser; and b. passing thebeam through a collimating lens and directly to an acousto-opticmodulator, wherein the acousto-optic modulator is electronicallycontrolled to generate an acoustic wave wherein the acoustic wave isconfigured to generate a diffracted optical beam having a desiredwavelength, wherein the diffracted optical beam is provided by a pair oftraveling waves from two acousto-optic modulators oppositely oriented tocancel out a frequency Doppler shift produced by a first acousto-opticmodulator of the two acousto-optic modulators.
 20. (canceled) 21.(canceled)
 22. The method of claim 21, wherein a tunable output beam isextracted from the laser cavity from the front facet of the laser cavityhaving a partially reflecting coating.
 23. The method of claim 21,wherein a tunable output beam is extracted from the laser cavity from antransmitted undiffracted beam, with a front facet of the laser cavityhaving a high reflection coating.
 24. The method of claim 19, whereinthe diffracted beam is generated by a standing wave.
 25. The method ofclaim 24, wherein a tunable output beam is extracted from the lasercavity from a front facet having a partially reflecting coating.
 26. Themethod of claim 24, wherein a tunable output beam is extracted from thelaser cavity from an undiffracted beam, with a front facet of the laserhaving a high reflection coating.